This book interrogates the role of quantification in stories on social media: how do visible numbers (e.g. of views, shares, likes) and invisible algorithmic measurements shape the stories we post and engage with? The links of quantification with stories have not been explored sufficiently in storytelling research or in social media studies. This is despite the fact that platforms have been integrating sophisticated metrics into developing facilities for sharing stories, with a massive appeal to ordinary users, influencers and businesses alike. With case-studies from Instagram, Reddit and Snapchat, we show how three types of metrics, namely content metrics, interface metrics and algorithmic metrics, affect the ways in which cancer patients share their experiences; the circulation of specific stories that mobilize counter-publics; the design of stories as facilities on platforms. Our analyses document how numbers structure elements in stories, indicate and produce engagement and become resources for the tellers’ self-presentation.